Hello all. I know some of you have been waiting for my report on the ASI183MM Pro. I've been working with it for a few months now, and have data for several images. So far it's been entirely narrow band, however I have started acquiring LRGB data on M33. I am still evaluating the camera, both from a normal usage standpoint, as well as from a low level characteristic standpoint. So more will come.
I am purposely starting this thread as a generic test thread for Sony IMX183 based cameras. Doesn't matter all that much to me which brand, all of them need testing, and with CMOS sensors so much of the functionality is integrated into the sensor, most of the differences between the cameras are pretty minor (especially these days, as ZWO has caught up to QHY and Atik in terms of standardized camera build and standard features like DDR memory buffer and the like.) So feel free to talk about any brand in this thread, I encourage it. The key is the underlying sensor, the IMX183 mono (there is also a color version, discussion about that sensor is also welcome, and I know there is a QHY183C on the market already, and I think it has been given too little attention...I've tested some data from that camera as well, and I thought it was quite good.)
The Sensor
Starting with the sensor itself. This is a new largish monochrome CMOS sensor. It's the second such sensor to find it's way into astro cameras since the ASI1600, which was the first mono CMOS sensor larger than a fractional inch size to be placed into an astro cam. The Sony IMX183CLK is slightly smaller than the Panasonic MN34230ALJ, the latter being a micro 4/3 format sensor (1.33x as long as tall for the aspect ratio), the former being a standard 1" format (1.5x as long as tall for the 3/2 aspect ratio). The Sony sensor has 2.4 micron pixels, compared to the Panasonic's 3.8 micron pixels, making the Sony pixel area about 2.5x smaller. The Sony sensor has 20 megapixels, compared to the Panasonic's 16 megapixels. Image dimensions are 5496x3672. FITS file size is just about 40 megs.
Dark current is very low, possibly lower on the Sony sensor than the Panasonic sensor. Like most CMOS sensors, it has amp glow, and this makes getting an accurate measurement of the dark current of the sensor more difficult, as true dark current generally grows at a different rate than the amp glows grow. Additionally dark current growth is usually linear, while amp glow growth tends to be non-linear as well as non-uniform across the sensor. Testing is still being performed to get a more accurate read of the real world dark current for this sensor, as well as to characterize the dark current separately from the amp glow, at least to some degree.
The rated dark current rate for the IMX183CLK is ~0.002e-/s @ -20C. In comparison to the MN34230ALJ which is rated at ~0.006e-/s @ -20C. Read noise for the IMX183CLK ranges from ~2.9e- at minimum gain (0), to as little as ~1.4e- at maximum gain (270, asi). This is a similar range to the MN34230ALJ, which has read noise ranging from ~3.5e- at minimum gain (0), to as little as 1.1e- at maximum gain (300, asi).
Initial Conclusion
With such small pixels, but a reasonably largish sensor, it puts the IMX183 based cameras in an interesting position for high resolution imaging. The pixels are the smallest you can get right now in reasonably sized sensors, making it the highest resolution sensor on the market alongside the IMX178. This has interesting implications for both high resolution imaging of smaller objects such as galaxies, but at more moderate focal lengths of around 800-1000mm, as well as for better-sampled imaging of larger objects, nebula, molecular clouds, etc. at shorter focal lengths of 135-400mm. One of the toughest things about wide field imaging is poor image scale, which leads to very bloated stars that can often be difficult to manage. The IMX183 may shine most as a high resolution wide field imager, and in general as a decent high resolution imager for efficient, smaller and more cost effective telescopes (i.e. instead of needing a C14Edge with a KAF-16803 to get decent high resolution images, you could use a much more cost effective 8" f/4 newt, and get similar results to the SCT/KAF setup, but in less time).
Read Noise
On the read noise front, it is important to note that these are the per-pixel values. Because the IMX183 has smaller pixels, the read noise in apples-to-apples terms is actually higher than that of the MN34230ALJ. When normalizing the pixel size to 3.8 microns, the IMX183 has an effective max read noise of about 4.5e-/3.8um, and effective minimum read noise of about 2.2e-/3.8um.
This was a slightly surprising discovery to me. My original expectations of the ASI183MM Pro were that it would have a 14-bit ADC, much like the ASI178 which has similar pixel size, as well as similar read noise to the ASI178. The ASI178 has read noise ranging from about 2.25e- down to about 1.3e-. The ASI183MM Pro ended up having a 12-bit ADC, which adds some additional quantization error, which is another noise term rolled into the total read noise, and generally accounts for the differences. However, with the smaller pixels, it does make the camera a slightly higher noise camera, in line with the newer Sony ICX CCD sensors.
Due to the higher read noise, the IMX183 benefits from longer exposures than are necessary with the MN34230ALJ. This is not an issue with LRGB, and even with exposures doubled or tripled compared to the MN34230ALJ, exposure times are still entirely reasonable. For some people, the longer LRGB exposures may even be much preferred, for those who don't like short exposures or don't want to stack hundreds of LRGB subs. This should make the IMX183 cameras a very attractive option for galaxy imagers looking to maximize resolution on a more reasonable budget. I think an 8-10" f/4 newt, or refractors in the 130-180mm f/6-f/7 range, paired with the IMX183 Mono could deliver amazing results for galaxies.
Narrow band is a slightly different story. Where the MN34230ALJ can handle 3-5 minute subs at a higher gain and effectively swamp the read noise, the IMX183 has more limited DR at high gain, and swamping the read noise becomes more of a challenge. In my testing with narrow band, I have found that gain 53 (asi) combined with 7-10 minute subs at f/4 delivers better results for narrow band. The SNR per sub is better, DR is quite good. This is again in line with Sony ICX CCD sensors which have similar amounts of read noise in a normalized 3.8 micron effective pixel. One might think of the IMX183 as the closest CMOS counterpart to a Sony ICX CCD, in my experience the results are very similar (main comparisons have been ICX814 data I've worked with in recent months.)
Calibrated IMX183 300s dark frame, starburst glow area, to show noise
Amp Glow
Like most consumer grade CMOS sensors, the IMX183 has amp glow. This shouldn't be surprising to anyone, however it may be interesting to know the differences and similarities with other sensors. The IMX183 is a Sony sensor, unlike the MN34230ALJ which is a Panasonic sensor. Sony sensors have a fairly consistent amp glow characteristic, an the IMX183 shares it.
The glow characteristic is two radial glows in the lower left and right corners, small, covering only the corners, plus a starburst glow (a multi-rayed glow that sends rays off across the area of the sensor) off to the right edge. There is a very slight and large scale glow to the upper left that is often imperceptible outside of deep stacks of darks. This is the same as those familiar with some of the other Sony IMX sensors, such as the IMX178, IMX294, etc.
In contrast, the Panasonic glows are a very faint large scale glow to the upper left, and the double-bubble along the right edge. The double bubble is actually more like a half doughnut shape, once you've stacked a decent number of darks into a master.
There is another glow characteristic difference between the Sony and Panasonic sensors. The Panasonic sensors have a larger read-time dependent factor in how bright the glows get than the Sony sensors, and a smaller exposure-time dependent factor. Sony sensors may have a slight read-time dependency, however they appear to be primarily dependent on the length of the exposure. As exposure time grows, so do the glows. With bright signals, the glows can be effectively buried, so with either camera they are usually a non-issue for LRGB or OSC imaging.
With fainter signals and narrow band, the glows of the IMX183 could potentially be a bigger issue. Narrow band imaging is very effective with the IMX183 (most of the images I'll be sharing so far have been narrow band), however with longer exposures of 10 minutes or longer, the amp glows can leave a bit of extra noise in the corners and near the starburst. With deep stacks this isn't an issue, however for mosaicing, it could potentially leave some visible seams in those areas.
Calibration
Like any camera that has amp glow, best practice is to calibrate with well matched darks (same gain, offset, temp and exposure time), without any dark optimization/dark scaling. Glows calibrate out perfectly with proper calibration. The hot pixels on the Sony IMX183 appear to be much more stable than the hot pixels on the Panasonic MN34230ALJ. This makes dark frame subtraction far more effective in removing hot pixels with the IMX183 than the MN34230ALJ.
The Panasonic sensor appears to have more RTS (random telegraph signal) which leads to semi-fixed hot pixels. RTS noise can result in certain pixels oscillating between two or three (or even more) relatively fixed states. These are not hot pixels in the classic sense, but they look the same in a sub exposure. The problem with RTS pixels is they are inconsistent. The same pixels do not always exhibit the same way in every frame, so even with dark subtraction, many "hot pixels" may remain in the data. This makes dithering doubly important with the Panasonic sensor. Additionally, it can make Cosmetic Correction a very useful step, but that adds more work to the pre-processing workflow.
The IMX183 does not appear to need any additional cosmetic correction step after simple dark subtraction. After dark subtraction, I actually find the noise characteristic of the IMX183 is extremely pleasing, very clean, cleaner even than the Panasonic MN34230ALJ. It has a very CCD-like quality to it once calibrated, which should please many people. There is some slight banding that can appear at lower gain settings, so dithering is still highly recommended. In general, I recommend dithering with all cameras, regardless of the sensor type or usage pattern. FPN can exhibit in many ways, even spatially random noise, and dithering will always benefit the final integrations by ensuring that any patterns are randomized through the stack. Utilize dithering with the IMX183 as you would any other camera.
Release Dates and Scope Pairings
I don't have any concrete information on release dates for these cameras. What I have heard is they are coming soon, though. End of November to early December, and they should be hitting the street. From both ASI and QHY, as I understand, although who will hit the streets first I cannot say. I do not know if Atik is working on a camera with this sensor (I did ask, but they like to keep their cameras under wraps until they can get them out for beta testing), nor do I know if anyone else is working on one. For those interested in one of these cameras, keep an eye out.
The IMX183 is a very high resolution sensor. It's got ultra tiny pixels. This means you can get more out of the resolution of any given scope than with most other cameras...but it also assumes you have the seeing to support the kind of resolution this sensor can deliver as well. For those interested in this camera, if you truly have an average of 3" seeing, then you may not benefit from the resolution potential, and you may be better off with a camera using the Panasonic sensor. For those who have around 2" seeing or better, then you should indeed be able to benefit from the resolution this camera has to offer.
Very small ~1.7" FWHM stars on a night of good seeing in Colorado, ASI183MM Pro @ 600mm fl, f/4
For telescope pairings. Because of the tiny pixels and 1" sensor size, I would avoid pairing it with larger scopes. There is no need to use an RC or SCT with this sensor. Doing so would result in rather ridiculous image scales of 0.3"/px (8" RC) to as little as 0.12"/px (14" SCT)!! If you live in Paranal, and are slapping one of these cameras on something like an f/2 2-meter aperture scope, then you might be set! ;P However, I recommend shorter/smaller scopes.
I think for high resolution LRGB galaxy imaging, the ideal pairings would be with 8" or 10" newtonins, f/4 to f/5. That will deliver an image scale of about 0.5-0.6"/px, which would sample 2" seeing limited stars by 3.3-4x. Right in the sweet spot for optimal high resolution imaging, IMO. The f/4 or f/5 f-ratio should deliver photons quickly, giving you read noise swamping subs in around 60-90 seconds or so for L, and 2-3x that for RGB (gain 53, asi). I also think you could easily pair this with refractors in the 130-150mm aperture range, at f/6-f/7. That will deliver an image scale around 0.51-0.65"/px. A TEC140, for example, is 980mm long at f/7, should make for a pretty awesome scope for high res galaxies if you've got the seeing for it.
For wide field narrow band, I think the ideal focal length range is going to be from around 200mm, up through about 400mm. This will give image scales in the range of ~2.4"/px to around 1.2"/px. For wide field nebular imaging, that is again pretty much the sweet spot. At 2.4"/px, your sampling ratio for 2" seeing would be closer to 1x, which is a lot better than say 5.4 micron pixels which would have a tiny image scale of 5.5"/px @ 200mm! You should be able to avoid terribly blocky stars with around a 1x sampling. At 400mm you would be sampling 2" seeing at around 1.7x, which should ensure you don't have blocky stars. This should also render stars at a more reasonable size, so that with wide fields the stars don't totally dominate the image (often a problem with image scales of 3"/px or smaller.) At such image scales, narrow band imaging becomes a totally viable option, thoroughly swamping the read noise should only require exposures of a few minutes. I'm quite curious to see what kinds of large scale mosaics could be created at 400mm, without being so totally dominated by stars, and with higher resolution and more details...should be quite interesting!!
In final conclusion, don't expect the IMX183 to be an "ASI1600 killer" in any way. The IMX183 is actually a slightly higher noise camera. It's a camera with a different purpose. With such tiny pixels, it has exciting implications for high res imaging with much smaller scopes than have classically been required, without the need to spend the big bucks on very large cameras that have very large pixels. With LRGB imaging, read noise won't matter much, however the smaller pixel size could deliver much more detailed images than any other camera at any given focal length (depends on seeing). It could also present some interesting implications for true lucky imaging, separating double stars, lucky imaging of small planetary nebula with a more budget-friendly telescope, etc.
Well, that's it for now. I have some example images to share, some low level dark and bias frames and comparisons to share, and some noise and gain evaluations in the works. I'll post more about those over the coming week.
Edited by Jon Rista, 22 November 2017 - 04:14 PM.
I'll be sharing some example images today, then some time this weekend and next week I hope to be sharing at least some examples of dark and bias frames and the like, along with some analyses. I'm working on a PTC analysis since I'm having trouble determining the full noise characteristic jus with PI standard measurements.
